Method and system for accurate gain adjustment of a transimpedance amplifier using a dual replica and servo loop

ABSTRACT

Methods and systems for accurate gain adjustment of a transimpedance amplifier using a dual replica and servo loop is disclosed and may include, in a transimpedance amplifier (TIA) circuit comprising a first TIA, a second TIA, and a third TIA, each comprising a configurable feedback impedance, and a control loop, where the control loop comprises a gain stage with inputs coupled to outputs of the first and second TIAs and an output coupled to the configurable feedback impedance of the second and third TIAs: configuring a gain level of the first TIA by configuring its feedback impedance, configuring a gain level of the third TIA by configuring a reference current applied to an input of the first TIA, and amplifying a received electrical signal to generate an output voltage utilizing the third TIA. The reference current may generate a reference voltage at one of the inputs of the gain stage.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.14/997,938 filed on Jan. 18, 2016, which claims priority to and thebenefit of U.S. Provisional Application 62/125,290 filed on Jan. 16,2015, which is hereby incorporated herein by reference in its entirety.

FIELD

Certain embodiments of the disclosure relate to semiconductor photonics.More specifically, certain embodiments of the disclosure relate to amethod and system for accurate gain adjustment of a transimpedanceamplifier using a dual replica and servo loop.

BACKGROUND

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present disclosure as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY

A system and/or method for accurate gain adjustment of a transimpedanceamplifier using a dual replica and servo loop, substantially as shown inand/or described in connection with at least one of the figures, as setforth more completely in the claims.

Various advantages, aspects and novel features of the presentdisclosure, as well as details of an illustrated embodiment thereof,will be more fully understood from the following description anddrawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith accurate gain adjustment of transimpedance amplifiers using a dualreplica and servo loop, in accordance with an example embodiment of thedisclosure.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure.

FIG. 2 is a block diagram of a photodiode sensed using a transimpedanceamplifier with feedback impedance, in accordance with an exampleembodiment of the disclosure.

FIG. 3 illustrates a transimpedance amplifier with accurate gainadjustment using a dual replica and servo loop, in accordance with anexample embodiment of the disclosure.

FIG. 4 is a flow chart illustrating example steps in providing accurategain adjustment for a transimpedance amplifier using a dual replica andservo loop, in accordance with an example embodiment of the disclosure.

DETAILED DESCRIPTION

Certain aspects of the disclosure may be found in a method and systemfor accurate gain adjustment of a transimpedance amplifier using a dualreplica and servo loop. Exemplary aspects of the disclosure maycomprise, in a transimpedance amplifier (TIA) circuit comprising a firstTIA, a second TIA, and a third TIA, each comprising a configurablefeedback impedance, and a control loop, where the control loop comprisesa gain stage with inputs coupled to outputs of the first and second TIAsand an output coupled to the configurable feedback impedance of thesecond and third TIAs: configuring a gain level of the first TIA byconfiguring its feedback impedance, configuring a gain level of thethird TIA by configuring a reference current applied to an input of thefirst TIA, and amplifying a received electrical signal to generate anoutput voltage utilizing the third TIA. The reference current maygenerate a reference voltage at one of the inputs of the gain stage. Thereference voltage may control the output of the gain stage that iscoupled to the configurable feedback impedance of the third TIA. Theconfigurable impedance of the first TIA may comprise a configurablearray of resistors and the configurable impedances of the second andthird TIAs may each comprise a resistor and a transistor. Theconfigurable impedances of the second and third TIAs may be configuredby configuring a gate voltage of the transistor utilizing the output ofthe gain stage. The output voltage may be coupled to a first input of asecond gain stage wherein a second input of the second gain stage iscoupled to an output of a fourth TIA. A differential signal may begenerated utilizing the output voltage and an output of the second gainstage. The output of the second gain stage may be configured byconfiguring an input current of the third TIA. The electrical signal maybe received from a photodetector.

FIG. 1A is a block diagram of a photonically-enabled integrated circuitwith accurate gain adjustment of transimpedance amplifiers using a dualreplica and servo loop, in accordance with an example embodiment of thedisclosure. Referring to FIG. 1A, there are shown optoelectronic deviceson a photonically-enabled integrated circuit 130 comprising opticalmodulators 105A-105D, photodiodes 111A-111D, monitor photodiodes113A-113D, and optical devices comprising couplers 103A-103C and gratingcouplers 117A-117H. There are also shown electrical devices and circuitscomprising amplifiers 107A-107D, analog and digital control circuits109, and control sections 112A-112D. The amplifiers 107A-107D maycomprise transimpedance and limiting amplifiers (TIA/LAs), for example.

In an example scenario, the photonically-enabled integrated circuit 130comprises a CMOS photonics die with a laser assembly 101 coupled to thetop surface of the IC 130. The laser assembly 101 may comprise one ormore semiconductor lasers with isolators, lenses, and/or rotators fordirecting one or more continuous-wave (CW) optical signals to thecoupler 103A. A CW optical signal may comprise an unmodulated opticalsignal comprising a coherent frequency component at a wavelength λ₁, forexample. The photonically enabled integrated circuit 130 may comprise asingle chip, or may be integrated on a plurality of die, such as withone or more electronics die and one or more photonics die.

Optical signals are communicated between optical and optoelectronicdevices via optical waveguides 110 fabricated in thephotonically-enabled integrated circuit 130. Single-mode or multi-modewaveguides may be used in photonic integrated circuits. Single-modeoperation enables direct connection to optical signal processing andnetworking elements. The term “single-mode” may be used for waveguidesthat support a single mode for each of the two polarizations,transverse-electric (TE) and transverse-magnetic (TM), or for waveguidesthat are truly single mode and only support one mode. Such one mode mayhave, for example, a polarization that is TE, which comprises anelectric field parallel to the substrate supporting the waveguides. Twotypical waveguide cross-sections that are utilized comprise stripwaveguides and rib waveguides. Strip waveguides typically comprise arectangular cross-section, whereas rib waveguides comprise a rib sectionon top of a waveguide slab. Of course, other waveguide cross sectiontypes are also contemplated and within the scope of the disclosure.

In an example scenario, the couplers 103A-103C may comprise low-lossY-junction power splitters where coupler 103A receives an optical signalfrom the laser assembly 101 and splits the signal to two branches thatdirect the optical signals to the couplers 103B and 103C, which splitthe optical signal once more, resulting in four roughly equal poweroptical signals.

The optical power splitter may comprise at least one input waveguide andat least two output waveguides. The couplers 103A-103C shown in FIG. 1Aillustrate 1-by-2 splitters, which divide the optical power in onewaveguide into two other waveguides evenly. These Y-junction splittersmay be used in multiple locations in an optoelectronic system, such asin a Mach-Zehnder interferometer (MZI) modulator, e.g., the opticalmodulators 105A-105D, where a splitter and a combiner are needed, sincea power combiner can be a splitter used in reverse.

The optical modulators 105A-105D comprise Mach-Zehnder or ringmodulators, for example, and enable the modulation of thecontinuous-wave (CW) laser input signal. The optical modulators105A-105D may comprise high-speed and low-speed phase modulationsections and are controlled by the control sections 112A-112D. Thehigh-speed phase modulation section of the optical modulators 105A-105Dmay modulate a CW light source signal with a data signal. The low-speedphase modulation section of the optical modulators 105A-105D maycompensate for slowly varying phase factors such as those induced bymismatch between the waveguides, waveguide temperature, or waveguidestress and is referred to as the passive phase, or the passive biasingof the MZI.

In an example scenario, the high-speed optical phase modulators mayoperate based on the free carrier dispersion effect and may demonstratea high overlap between the free carrier modulation region and theoptical mode. High-speed phase modulation of an optical mode propagatingin a waveguide is the building block of several types of signal encodingused for high data rate optical communications. Speed in the severalGb/s may be required to sustain the high data rates used in modernoptical links and can be achieved in integrated Si photonics bymodulating the depletion region of a PN junction placed across thewaveguide carrying the optical beam.

One output of each of the optical modulators 105A-105D may be opticallycoupled via the waveguides 110 to the grating couplers 117E-117H. Theother outputs of the optical modulators 105A-105D may be opticallycoupled to monitor photodiodes 113A-113D to provide a feedback path. TheIC 130 may utilize waveguide based optical modulation and receivingfunctions. Accordingly, the receiver may employ an integrated waveguidephoto-detector (PD), which may be implemented with epitaxialgermanium/SiGe films deposited directly on silicon, for example.

The grating couplers 117A-117H may comprise optical gratings that enablecoupling of light into and out of the photonically-enabled integratedcircuit 130. The grating couplers 117A-117D may be utilized to couplelight received from optical fibers into the photonically-enabledintegrated circuit 130, and the grating couplers 117E-117H may beutilized to couple light from the photonically-enabled integratedcircuit 130 into optical fibers. The grating couplers 117A-117H maycomprise single polarization grating couplers (SPGC) and/or polarizationsplitting grating couplers (PSGC). In instances where a PSGC isutilized, two input, or output, waveguides may be utilized.

The optical fibers may be epoxied, for example, to the CMOS chip, andmay be aligned at an angle from normal to the surface of thephotonically-enabled integrated circuit 130 to optimize couplingefficiency. In an example embodiment, the optical fibers may comprisesingle-mode fiber (SMF) and/or polarization-maintaining fiber (PMF).

In another exemplary embodiment illustrated in FIG. 1B, optical signalsmay be communicated directly into the surface of thephotonically-enabled integrated circuit 130 without optical fibers bydirecting a light source on an optical coupling device in the chip, suchas the light source interface 135 and/or the optical fiber interface139. This may be accomplished with directed laser sources and/or opticalsources on another chip flip-chip bonded to the photonically-enabledintegrated circuit 130.

The photodiodes 111A-111D may convert optical signals received from thegrating couplers 117A-117D into electrical signals that are communicatedto the amplifiers 107A-107D for processing. In another embodiment of thedisclosure, the photodiodes 111A-111D may comprise high-speedheterojunction phototransistors, for example, and may comprise germanium(Ge) in the collector and base regions for absorption in the 1.3-1.6 μmoptical wavelength range, and may be integrated on a CMOSsilicon-on-insulator (SOI) wafer and/or die.

The analog and digital control circuits 109 may control gain levels orother parameters in the operation of the amplifiers 107A-107D, which maythen communicate electrical signals off the photonically-enabledintegrated circuit 130. The control sections 112A-112D compriseelectronic circuitry that enables modulation of the CW laser signalreceived from the splitters 103A-103C. The optical modulators 105A-105Dmay require high-speed electrical signals to modulate the refractiveindex in respective branches of a Mach-Zehnder interferometer (MZI), forexample.

In operation, the photonically-enabled integrated circuit 130 may beoperable to transmit and/or receive and process optical signals. Opticalsignals may be received from optical fibers by the grating couplers117A-117D and converted to electrical signals by the photodetectors111A-111D. The electrical signals may be amplified by transimpedanceamplifiers in the amplifiers 107A-107D, for example, and subsequentlycommunicated to other electronic circuitry, not shown, in thephotonically-enabled integrated circuit 130.

Transimpedance amplifiers (TIAs) may be operable to receive electricalcurrent signals, amplify, and convert them to voltage signals. Inoptical transceivers, tight gain calibration limits may be imposed onthe TIAs, which usually limit the receiver dynamic range (DR), and atdata rates beyond 10 GB/s and with input photo current levels in a 30 dBrange, linear TIA performance adequate for optoelectronics applicationsis challenging. The TIA dynamic range may be defined as the ratio ofmaximum to minimum photocurrent that can be properly sensed.

The TIA linearity limits the receiver cascade dynamic range for largeinput currents. In scaled CMOS technologies, and under large level inputcurrents, it may be difficult to prevent TIA output saturation due inpart to the nonlinearity of the short channel transistors, but mainly tothe reduced supply voltages. The low end of the dynamic rage isdetermined by the TIA input referred noise. A low-noise topology TIA isnecessary in order to meet input sensitivity.

A shunt-feedback (SF)-based TIA may provide such a low-noise topology.This architecture is based on a voltage-inverting amplifier with afeedback network. The feedback network impedance directly affects thedynamic range of the TIA and gain may be controlled using a DC controlvoltage generated in a servo loop using two TIA replicas.

In an example scenario, the TIA topology disclosed herein may controlthe value of the feedback network impedance, thus enhancing the TIAlinearity under large levels of input photocurrent. The gain controlmethod is digital, non-intrusive, i.e. does not affect the signal path,and may be integrated with other process calibration and temperaturecontrol mechanisms.

FIG. 1B is a diagram illustrating an exemplary photonically-enabledintegrated circuit, in accordance with an example embodiment of thedisclosure. Referring to FIG. 1B, there is shown thephotonically-enabled integrated circuit 130 comprising electronicdevices/circuits 131, optical and optoelectronic devices 133, a lightsource interface 135, a chip front surface 137, an optical fiberinterface 139, CMOS guard ring 141, and a surface-illuminated monitorphotodiode 143.

The light source interface 135 and the optical fiber interface 139comprise grating couplers, for example, that enable coupling of lightsignals via the CMOS chip surface 137, as opposed to the edges of thechip as with conventional edge-emitting/receiving devices. Couplinglight signals via the chip surface 137 enables the use of the CMOS guardring 141 which protects the chip mechanically and prevents the entry ofcontaminants via the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theamplifiers 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the couplers103A-103C, optical terminations, grating couplers 117A-117H, opticalmodulators 105A-105D, high-speed heterojunction photodiodes 111A-111D,and monitor photodiodes 113A-113D.

In an example scenario, the electronic devices/circuits 131 may compriseTIAs with accurate gain control using dual replicas and a servo loop.Accordingly, a feedback impedance for the TIA may directly affect thedynamic range of the TIA and gain may be controlled using a DC controlvoltage generated in the servo loop using two TIA replicas.

FIG. 1C is a diagram illustrating a photonically-enabled integratedcircuit coupled to an optical fiber cable, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 1C, there is shown thephotonically-enabled integrated circuit 130 comprising the chip surface137 and the CMOS guard ring 141. There are also shown a fiber-to-chipcoupler 145, an optical fiber cable 149, and an optical source assembly147.

The photonically-enabled integrated circuit 130 comprises the electronicdevices/circuits 131, the optical and optoelectronic devices 133, thelight source interface 135, the chip surface 137, and the CMOS guardring 141 may be as described with respect to FIG. 1B.

In an example embodiment, the optical fiber cable may be affixed, viaepoxy for example, to the CMOS chip surface 137. The fiber chip coupler145 enables the physical coupling of the optical fiber cable 149 to thephotonically-enabled integrated circuit 130.

FIG. 2 is a block diagram of a photodiode sensed using a transimpedanceamplifier with feedback impedance, in accordance with an exampleembodiment of the disclosure. Referring to FIG. 2, there is shownreceiver front-end 200 comprising a photodetector 201, feedbackimpedance 203, and a gain stage 205. The photodetector may comprise asemiconductor photodiode, for example, comprising silicon orsilicon-germanium for receiving optical signals and converting them toan electrical current. The photodetector 201 may be on the same chip,such as the chip 130, as the gain stage 205 or may be off-chip.

The gain stage 205 may comprise circuitry for receiving an electricalcurrent signal at its input and providing a voltage at its output thatis proportional to the input current. In an example scenario, the gainstage 205 may comprise a voltage-in-voltage-out gain stage and maycomprise a CMOS inverter, although the disclosure is not so limited, asany gain stage compatible with the source signal may be utilized.

The feedback impedance 203 may comprise a feedback path for the gainstage 205, and may be implemented in a passive or active mode, withthree examples shown by feedback impedances 203A-203C. Feedbackimpedance 203A comprises a passive impedance with a fixed resistor,while feedback impedances 203B and 203C represent active impedances,where impedance 203B is a resistor in series with a transistor. Thetransistor may comprise a PMOS, NMOS, or other type of transistor, withthe control terminal of the transistor being utilized to configure theimpedance of the series structure. In an example scenario, an NMOS/PMOStransistor may be utilized and the resistance controlled by varying thegate voltage when operating in its linear region, or triode mode.

Similarly, the feedback impedance 203C comprises an array of selectableresistors, such as a trimmed resistor network. In this example, discreteresistors may be selected utilizing an array of switches coupled to theresistors in the network, as indicated by the input terminal.

The feedback impedance 203 and gain stage 205 may be utilized in TIAimplementations, as show further with respect to FIG. 3.

FIG. 3 illustrates a transimpedance amplifier with accurate gainadjustment using a dual replica and servo loop, in accordance with anexample embodiment of the disclosure. Referring to FIG. 3, there isshown receiver 300 comprising a photodetector 301, TIAs 310A-310D, andamplifiers 311 and 313. There is also shown an optical input signal, areference voltage V_(ref) at the output of TIA 310A, a control currentI_(ctrl) for, TIA 310C, a servo loop voltage V_(g), and an outputvoltage V_(out).

In an example scenario, the TIAs 310A-310D are substantially similar tothe TIA described with respect to FIG. 2, as indicated by the symbolswithin the boxes for each TIA, where the feedback impedance isconfigured depending on the function of the TIA. For example, TIA 310Amay comprise a trimmed resistor network, such as feedback impedance203C, while TIAs 310B, 310C, and 310D each comprise an active feedbackimpedance, such as feedback impedance 203B. In this manner, the gain ofTIA 310A, once selected, may remain essentially constant providing astable and known gain, while that of TIAs 310B, 310C, and 310D may beconfigured dynamically using a control voltage, the servo loop voltageV_(g). The TIAs 310B and 310D may be replicas of TIA 310C, withnominally identical components being located in close proximity on thechip, for example.

The amplifiers 311 and 313 may comprise operational amplifiers ordigital feedback, for example, with circuitry that generates an outputsignal proportional to the difference between its input voltages. Foramplifier 311, therefore, the output voltage Vg, is proportional to thedifference between the output of TIA 310A, Ref, and the output of TIA310B.

The TIA circuit comprising TIAs 310A-310D in the receiver 300 maycomprise a CMOS shunt-feedback TIA, whose low-frequency gain(trans-impedance ZT) is correlated to the feedback network impedance.This approximation is correct for a large enough gain for the gainstages in the TIAs 310A-310D, A>10, for example. For A>10, ZT isapproximately equal to the feedback impedance.

The main TIA, TIA 310C, and the two TIA replicas, TIAs 3108 and 310D inthis example, comprise a voltage-controlled feedback impedance,comprising a series combination of a voltage-controlled resistance and afixed resistor, as shown by impedance 203B in FIG. 2, where thetriode-mode n-MOS transistor creates the voltage control portion of theimpedance and the poly resistor portion further linearizes theimpedance, under overload input signals, as well as compensates then-MOS temperature coefficient.

The main TIA 310C and TIA replicas 310B and 310D are configured by theservo loop voltage V_(g), which is coupled to the voltage control inputsof the feedback impedances of the TIAs, thereby providing an accuraterepresentation of a trimmed target feedback impedance. This controlvoltage V_(g) imparts the same trimmed impedance value to the impedancenetworks in the TIA 310C and TIA replicas 310B and 310D, within processand layout matching limits.

The impedance resistance trim servo loop employs a reference TIA 310Athat generates an output voltage proportional to I_(ref) x targetimpedance. In an example scenario, I_(ref) and I_(ref1) are small fixedcurrents, generated by a current digital-to-analog converter (DAC),resulting in a known and accurate current. I_(ref) and I_(ref1) may beinjected into TIA 310A and TIA 310B reference inputs, respectively, as ameans to increase the matching between the four TIAs, as well as a meansto control the TIA 310C gain within a narrow range around the trimmedpoint.

The main TIA 310C gain may be controlled by the control voltage V_(g).This voltage in turn may be adjusted by changing the reference voltage(REF) in the servo-loop. REF is generated by the product of I_(ref) andthe transimpedance value of the TIA 310A with the trimmed resistancenetwork. The servo-loop equalizes the two voltages seen by the amplifier311, to the level of REF, generating the control voltage V_(g). Thisvoltage is proportional to the TIA 310B gain, and it imparts an equalvalue to the main TIA 310C gain, within matching limits.

Note that the above TIA gain trim approach described here does notaffect the high-speed operation of the main TIA 310C in the signal path,since the gain control is applied as a DC voltage on the gate of thefeedback n-MOS transistor. This control voltage V_(g) has a precisevalue correlated to the trimmed poly resistor, and a temperaturecoefficient that tracks the same resistor. Also, the control voltageV_(g) has a variable component, due to its dependence on I_(ref), thatis used to control the TIA 310C gain around the trimmed value.

In operation, an optical signal may be directed to the photodetector 301via an optical waveguide for example, and a resulting electrical signalmay be communicated to the TIA 310C. Reference currents I_(ref) andI_(ref1) may be supplied to the inputs of the TIAs 310A and 310B,respectively. The feedback impedance of the TIA 310A, comprising a trimresistor network, may be preselected to configure the gain, while thecontrol voltage V_(g) acts to equalize the gain of TIA 310B. This samecontrol voltage controls the gain of the main TIA 310C, which receivesthe electrical signal from the photodetector 301, resulting in anamplified signal Vout proportional to the gain of the TIA 310C.

The gain of the main TIA 310C may be adjusted by configuring I_(ref),such that the reference voltage V_(ref) changes and V_(g) adjustsaccordingly to equalize the inputs to the amplifier 311. This change inV_(g) thus changes the gain of TIA 310C.

In addition, a “pseudo-differential” output may be generated byconfiguring the output of the amplifier 313 to follow the average of theoutput signal V_(out). This may be accomplished by coupling the outputsignal V_(out) to one input of the amplifier 313 and coupling the outputof the TIA 310D to the other input of the amplifier 310D. By configuringthe current I_(ctrl) for the TIA 310C to sink or source current at theinput of the TIA, the DC value of the output voltage V_(out) may beadjusted, and with the input to the TIA 310D being open, the output ofthe amplifier 313 is therefore inverted from the change in DC value ofthe output voltage and provides a feedback control voltage, DC_(ctrl).The differential output is therefore defined by the outputs of the TIAs310C and 310D.

FIG. 4 is a flow chart illustrating example steps in providing accurategain adjustment for a transimpedance amplifier using a dual replica andservo loop, in accordance with an example embodiment of the disclosure.In step 403, after start step 401, the feedback impedance of TIA 310Amay be configured by selecting a resistance in a resistor network to setthe reference TIA gain. In step 405, reference currents I_(ref) andI_(ref1) may be applied to reference and replica TIAs 310A and 310B. Instep 407, an electrical signal may be received from the photodetector301, thereby generating an electrical signal, which may be amplified byTIA 310C in step 409. In addition, a control current may be supplied tothe main TIA 310C to provide a configurable DC output, thereby enablinga pseudo-differential output. In step 411, the amplified andpseudo-differential signal may be communicate at the output of the mainTIA 310C, followed by end step 611.

In an example embodiment, a method and system are disclosed for accurategain adjustment of a transimpedance amplifier using a dual replica andservo loop. In this regard, aspects of the disclosure may comprise atransimpedance amplifier (TIA) circuit comprising a first TIA, a secondTIA, and a third TIA, each comprising a configurable feedback impedance,and a control loop. The control loop comprises a gain stage with inputscoupled to outputs of the first and second TIAs and an output coupled tothe configurable feedback impedance of the second and third TIAs. TheTIA circuit is operable to: configure a gain level of the first TIA byconfiguring its feedback impedance, configure a gain level of the thirdTIA by configuring a reference current applied to an input of the firstTIA, and amplify a received electrical signal to generate an outputvoltage utilizing the third TIA.

The reference current may generate a reference voltage at one of theinputs of the gain stage. The reference voltage may control the outputof the gain stage that is coupled to the configurable feedback impedanceof the third TIA. The configurable impedance of the first TIA maycomprise a configurable array of resistors and the configurableimpedances of the second and third TIAs may each comprise a resistor anda voltage-controlled resistance.

The configurable impedances of the second and third TIAs may beconfigured by configuring a gate voltage of the transistor utilizing theoutput of the gain stage. The output voltage may be coupled to a firstinput of a second gain stage where a second input of the second gainstage is coupled to an output of a fourth TIA. A differential signal maybe generated utilizing the output voltage and an output of the secondgain stage. The output of the second gain stage may be configured byconfiguring an input current of the third TIA. The electrical signal maybe received from a photodetector.

In another example embodiment, a system is disclosed for accurate gainadjustment of a transimpedance amplifier using a dual replica and servoloop. In this regard, aspects of the disclosure may comprise atransimpedance amplifier (TIA) circuit, where the TIA circuit comprises:a first TIA comprising a first configurable impedance and an outputcoupled to a first input of a first gain stage, a second TIA comprisinga second configurable impedance and an output coupled to a second inputto the first gain stage, a third TIA comprising a third configurablefeedback impedance, an input for receiving an input electrical current,and an output that generates an output voltage of the TIA circuit, and acontrol loop for controlling the gain of the third TIA. The control loopcomprises the first gain stage with its output coupled to the second andthird configurable impedances.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry or a device is “operable” to perform afunction whenever the circuitry or device comprises the necessaryhardware and code (if any is necessary) to perform the function,regardless of whether performance of the function is disabled or notenabled (e.g., by a user-configurable setting, factory trim, etc.).

While the disclosure has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present disclosure. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present disclosure without departing from itsscope. Therefore, it is intended that the present disclosure not belimited to the particular embodiments disclosed, but that the presentdisclosure will include all embodiments falling within the scope of theappended claims.

What is claimed is:
 1. A method for communication, the methodcomprising: in a receiver comprising: a first transimpedance amplifier(TIA); a second TIA; a third TIA; a fourth TIA, each TIA comprising aconfigurable feedback impedance; a control loop, the control loopcomprising a gain stage with inputs coupled to outputs of the first andsecond TIAs and an output of the gain stage coupled to the configurablefeedback impedance of the second and third TIAs; and a second gain stagewith a first input coupled to an output of the third TIA and a secondinput coupled to an output of the fourth TIA: configuring a gain levelof the first TIA by configuring its feedback impedance; configuring again level of the third TIA by configuring a reference current appliedto an input of the first TIA; and amplifying a received electricalsignal to generate an output voltage utilizing the third TIA.
 2. Themethod according to claim 1, wherein the reference current generates areference voltage at one of the inputs of the gain stage.
 3. The methodaccording to claim 2, wherein the reference voltage controls the outputof the gain stage that is coupled to the configurable feedback impedanceof the third TIA.
 4. The method according to claim 1, wherein theconfigurable impedance of the first TIA comprises a configurable arrayof resistors.
 5. The method according to claim 1, wherein theconfigurable impedances of the second and third TIAs each comprise afixed resistor and a voltage-controlled resistance.
 6. The methodaccording to claim 5, comprising configuring the configurable impedancesof the second and third TIAs by configuring a gate voltage of atransistor of the voltage-controlled resistance utilizing the output ofthe gain stage.
 7. The method according to claim 1, comprisinggenerating a differential signal utilizing the output voltage and anoutput of the fourth TIA.
 8. The method according to claim 1, comprisingconfiguring the output of the second gain stage by configuring an inputcurrent of the third TIA.
 9. The method according to claim 1, comprisingreceiving an optical signal via a photodetector coupled to an input ofthe third TIA.
 10. The method according to claim 9, comprising receivingthe electrical signal generated by the optical signal received by thephotodetector.
 11. A system for communication, the system comprising: areceiver comprising a first transimpedance amplifier (TIA); a secondTIA; a third TIA; a fourth TIA, each TIA comprising a configurablefeedback impedance; a control loop, the control loop comprising a gainstage with inputs coupled to outputs of the first and second TIAs and anoutput coupled to the configurable feedback impedance of the second andthird TIAs; and a second gain stage with a first input coupled to anoutput of the third TIA and a second input coupled to an output of thefourth TIA, the receiver being operable to: configure a gain level ofthe first TIA by configuring its feedback impedance; configure a gainlevel of the third TIA by configuring a reference current applied to aninput of the first TIA; and amplify a received electrical signal togenerate an output voltage utilizing the third TIA.
 12. The systemaccording to claim 11, wherein the reference current generates areference voltage at one of the inputs of the gain stage.
 13. The systemaccording to claim 12, wherein the reference voltage controls the outputof the gain stage that is coupled to the configurable feedback impedanceof the third TIA.
 14. The system according to claim 11, wherein theconfigurable impedance of the first TIA comprises a configurable arrayof resistors.
 15. The system according to claim 11, wherein theconfigurable impedances of the second and third TIAs each comprise afixed resistor and a voltage-controlled resistance.
 16. The systemaccording to claim 15, wherein the TIA circuit is operable to configurethe configurable impedances of the second and third TIAs by configuringa gate voltage of a transistor of the voltage-controlled resistanceutilizing the output of the gain stage.
 17. The system according toclaim 11, wherein the received electrical is generated by aphotodetector coupled to an input of the third TIA.
 18. The systemaccording to claim 11, wherein the TIA circuit is operable to generate adifferential signal utilizing the output voltage and an output of thefourth TIA.
 19. The system according to claim 11, wherein the TIAcircuit is operable to configure the output of the second gain stage byconfiguring an input current of the third TIA.
 20. A system forcommunication, the system comprising: a receiver comprising: a firsttransimpedance amplifier (TIA) comprising a first configurable impedanceand an output coupled to a first input of a first gain stage; a secondTIA comprising a second configurable impedance and an output coupled toa second input to the first gain stage; a third TIA comprising a thirdconfigurable feedback impedance, an input for receiving an inputelectrical current, and an output that generates an output voltage ofthe TIA circuit; a fourth TIA comprising a fourth configurableimpedance; a second gain stage with a first input coupled to an outputof the third TIA and a second input of the second gain stage coupled toan output of a fourth TIA; and a control loop for controlling the gainof the third TIA, the control loop comprising the first gain stage withits output coupled to the second and third configurable impedances.